ARTICLE

Geology, alteration and lithogeochemistry of the PaleoproterozoicKorpela VMS occurrence in Eastern Finland

Janne Hokka1

Received: 8 March 2019 /Accepted: 1 January 2020# The Author(s) 2020

AbstractKorpela is a Cu-Zn-Au VMS occurrence hosted by bimodal, sub-alkaline, volcanic and volcaniclastic rocks. It is part of aSvecofennian supracrustal sequence 1.93–1.91 Ga in age. In this study, lithogeochemical evidence is used to assess the VMS-prospectivity in the Korpela area to serve regional-scale exploration and provide detailed information on chemostratigraphy andhydrothermal alteration within the Korpela succession. Korpela is understood to have been formed in an evolved arc rift, possiblyin a continental back-arc environment. The felsic rocks of the sequence are FII-FIIIa, HFSE-enriched (A-type) rhyolites overlainand, locally cross-cut, by mafic rocks with MORB/BABB signatures and felsic synvolcanic porphyry dykes. In the vicinity ofKorpela, tonalitic subvolcanic intrusions intrude the supracrustal rocks which share textures common with local shallow VMS-related intrusion complexes. The Korpela area comprises a volcanic succession where primary volcanic textures are completelydestroyed by multiple deformation, metamorphism and alteration. Using detailed volcanic chemostratigraphy established fromdownhole geochemical profiles, 12 chemostratigraphic units and 21 chemical rock types could be identified ranging from basaltto rhyolite. Several metamorphic mineral assemblages were identified which were further classified into six alteration types, i.e.Mg-Fe-S, K-Al-Fe-(± S), K-Al-Mg-Fe-S, K, Si-K-Ca-(± S) and Ca-(± Na), using a combination of mineralogy and geochemistry.The chemostratigraphy and alteration studies help in understanding the volcanic stratigraphy and in recognising a potential VMS-related alteration.

Keywords VMS deposits . Lithogeochemistry . Chemostratigraphy . Hydrothermal alteration . Paleoproterozoic . Finland

Introduction

Volcanogenic massive sulphide deposits (VMSs) are a glob-ally important source of base metals (Pb, Cu and Zn) and othermetals (Ag, Au, Cd, Se, Sn, Bi, Ge, Ga and In) (e.g. Franklinet al. 2005; Galley et al. 2007). The exploration and mininghistory for VMS deposits in Finland is long, dating back to theeighteenth century, whenmining began at the Orijärvi deposit,southern Finland (Haapala and Papunen 2015). Economically,the most significant VMS camp in Finland is the Vihanti-

Pyhäsalmi belt in the NW part of the Raahe-Ladoga ShearZone (RLSZ; Fig. 1), a collage of distinct shear zones, whereseven deposits with a reported mineral resource and more thanten occurrences have been discovered (e.g. Mäki et al. 2015).About 100 Mt of ore has been mined from the Vihanti-Pyhäsalmi belt since 1954 (Mäki et al. 2015). Despite severalnear surface discoveries, the RLSZ can nevertheless be con-sidered underexplored at depth. This is particularly true for theSE part of the RLSZwhich forms the focus of this study. In the1950s and 1960s, the exploration focus was in the Virtasalmisuite leading to one mine, Virtasalmi (Cu) (Fig. 1) which wasactive during the period 1966–1984 (Puustinen 2003). TheViholanniemi area, east of Virtasalmi (Fig. 1c), has witnesseda relatively small amount of exploration. The initial work con-ducted by the Geological Survey of Finland (GTK) in thenorthern part of the Viholanniemi suite led to the discoveryof the Viholanniemi Zn-Cu-Pb-Ag-Au deposit in 1985(Makkonen 1991). In the early 1990s, Outokumpu Oy ex-plored the southern part of the area, but exploration activitiessoon ceased due to poor results (Puustjärvi 1992).

Editorial handling: P. Eilu

Electronic supplementary material The online version of this article(https://doi.org/10.1007/s00126-020-00954-0) contains supplementarymaterial, which is available to authorized users.

* Janne Hokkajanne.hokka@gtk.fi

1 Geological Survey of Finland (GTK), P.O. Box 96,FI-02151 Espoo, Finland

Mineralium Depositahttps://doi.org/10.1007/s00126-020-00954-0

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In the early stages of exploration, it is critical to identify theVMS-prospective belts from the less prospective ones andlocate the most favourable lithological units within the volca-nic successions (e.g. Hodges and Manojlovic 1993; Galleyet al. 1993; Paulick et al. 2001). This requires an understand-ing of volcanic and chemical stratigraphy and hydrothermalalteration in order to target cost-effectively to the proximity ofore. It has been recognised that there are several critical char-acteristics in volcanic rocks, especially on felsic suites, thathelp to identify potentially VMS-prospective systems (e.g.Lesher et al. 1986; Lentz 1998; Barrett and MacLean 1999;Piercey et al. 2001; Piercey 2011). The rift-related and high-temperature magmatism signatures commonly found in FII,FIII and FIV felsic volcanic rocks can be used as belt-scalegeochemical indicators for VMS-prospective rhyolites. Thiswidely used classification discriminates between four types ofrhyolite: FI, FII, FIII, FIV (e.g. Lentz 1998; Hart et al. 2004;Piercey 2011). Recognition of HFSE-enriched (A-type) rhyo-lites, overlain by MORB/BABB affinities and felsicsynvolcanic porphyry dykes, indicate a VMS-potential rift-related environment (e.g. Lentz 1998; Piercey 2011). Othercommonly recognised features related to VMS deposits glob-ally are the associated large subvolcanic intrusive complexesthat typically occur in proximity to the deposits (e.g. Galley1996, 2003; Campbell et al. 1981; Large et al. 1996;Hannington et al. 2005; Piercey et al. 2008; Piercey 2011;Bailes et al. 2016).

In glaciated terrains, as in the Viholanniemi area, the pau-city of outcrop means that the details of geology and volcanicstratigraphy rely almost exclusively on drill hole informationthough information from geophysical surveys and till geo-chemistry is also used. All the Paleoproterozoic VMS depositsin Finland and elsewhere have been affected by deformationand regional metamorphism, resulting in recrystallisation ofthe original alteration minerals to variable metamorphic as-semblages (e.g. Roberts et al. 2004; Latvalahti 1979; Mäkiet al. 2015; Barrett et al. 2005; Caté 2016; Kampmann et al.2017; Hollis et al. 2018). This often leads to complex classi-fication of metamorphic mineral assemblages and difficulties

in discriminating between lithological units. The use of im-mobile element geochemistry is a widely recognised tool inthe reliable protolith study of igneous rocks, in determining ofmagmatic affinities and the quantitative estimates of mass,volume and mineralogical changes in hydrothermally alteredand metamorphosed sequences (e.g. MacLean and Barrett1993; Barrett and MacLean 1994; Hannington et al. 2003;Barrett et al. 2005; Mercier-Langevin et al. 2007). Hence,lithogeochemical methods have allowed for the validation ofgeologic interpretation with proper protolith andchemostratigraphic identification regardless of metamorphismand alteration. Lithogeochemistry has also proven to providereliable signatures of identifying rift environments and thepresence of high-temperature magmatism which are criticalfactors in VMS Mineral Systems and are used for identifyingprospective and barren areas for hosting VMS mineralisation(e.g. Lentz 1998; Hart et al. 2004; Gaboury and Pearson 2008;Piercey 2010, 2011).

Author’s opinion is that these methods have not beenutilised widely enough in brownfield or greenfield VMS ex-ploration in Finland. Lithogeochemistry, especially the use ofchemostratigraphy and mobile-element geochemistry, hasmainly been applied in the Vihanti-Pyhäsalmi belt, inCentral Finland (Mäki 1986; Roberts et al. 2003; Robertset al. 2004; Imaña et al. 2013; Mäki et al. 2015). The studyof mobile element ratios and concentrations related to theproximal zone of the Pyhäsalmi deposit (Mäki 1986) servedas a successful exploration tool resulting in the discovery ofMullikkoräme VMS deposit in 1987, within the Vihanti-Pyhäsalmi belt (Mäki 1986; Mäki et al. 2015).

Korpela, a recently discovered Cu-Zn-AuVMS occurrencein the Viholanniemi area (Figs. 1 and 2, and ESM Fig. 1), isassociated with intense alteration and overprinted by regionalmetamorphism and deformation. Immobile-elementlithogeochemical methods have been used in this study toverify protoliths and establish chemostratigraphic relationsderived from downhole geochemical profiles and petrograph-ic observations. Hydrothermal alteration is studied to classifydifferent alteration types which can be further modelled in 3Dusing mobile-element geochemistry to recognise their geo-chemical footprints (Hokka 2020). Many of the more recentlithogeochemical studies applied to metamorphosed hydro-thermal ore systems have either been undertaken in relationto an active mine or brownfield exploration targets wherelarge amounts of data is available (e.g. Schlatter et al. 2003;Roberts et al. 2003; Barrett et al. 2005; Imaña et al. 2005;Mercier-Langevin et al. 2007; Schlatter 2007; Mäki et al.2015; Mills et al. 2016; Caté 2016; Chmielowski et al. 2016;Kampmann et al. 2017; Hollis et al. 2018). Although here thetarget is in a greenfield area, the same methods are applied,with the following objectives: (1) describe the main geologi-cal, mineralogical and geochemical features of the target andhosting volcanic succession. (2) Provide lithogeochemical

�Fig. 1 General geology of the southeastern part of the Raahe-LadogaShear zone (RLSZ). a The Jäppilä-Virtasalmi block in Eastern Finlandbelongs to the older Svecofennian 1.93–1.91 Ga magmatic rocks and iscomparable with the Vihanti-Pyhäsalmi belt. b The study area is locatedwithin the southeastern corner of the Jäppilä-Virtasalmi block in theViholanniemi suite. c The Jäppilä-Virtasalmi block comprises twovolcanic suites, the Virtasalmi suite in the west and the Viholanniemisuite in the east. The only mine in the area is the Virtasalmi deposit,where a total of 4.2 Mt of ore was mined with an average grade of0.73% Cu from 1966 to 1983 (Puustinen 2003). The Virtasalmi depositis a polydeformed, stratabound synvolcanic hydrothermal exhalative Cuoccurrence metamorphosed at lower-amphibolite facies conditions.Geological maps are from the Geological Survey of Finland (GTK)digital bedrock database (Bedrock of Finland—DigiKP). Thecoordinates are according to EUREF-FIN (ETRS-TM35FIN; WGS-84)

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evidence of VMS prospectivity in the area to serve regional-scale exploration. (3) Use immobile-element geochemistry tounravel volcanic stratigraphy in deformed, metamorphosedand hydrothermally altered rocks.

Regional geology

The Fennoscandian shield consists of Proterozoic andArchean units. The Archean rocks dominate in the easternand northern part whereas Proterozoic rocks cover much ofthe rest of the shield (Koistinen et al. 2001). The latter com-prise ca. 1.9-Ga orogenic terrains of southern and westernFinland including volcanic-sedimentary and migmatiticgneiss belts and granitoid complexes (Fig. 1a). The VMS

deposits of central Finland are hosted by multiply deformedbimodal volcanic rocks within the NW-trending crustal-scalesuture zone referred to as the Raahe-Ladoga Zone (e.g.Korsman et al. 1997, Koistinen 1981; Ekdahl 1993; Mäkiet al. 2015) or the Raahe-Ladoga Shear Zone (RLSZ; e.g.Mikkola et al. 2018a) (Fig. 1a, b). The RLSZ hosts a set ofPaleoproterozoic arc complexes accreted together and againstthe Archean craton (Korsman et al. 1997). This collisionalsetting forms the part of the Finnish Svecofennian domain,and is bounded by the Archean basement with overlyingPaleoproterozoic (Karelian) sequences in the east and theCentral Finland Granitoid Complex (CFGC) in the west(Fig. 1a). The Svecofennian magmatic rocks in this area formtwo distinct age groups: an older 1.93–1.91 Ga and a younger

Tonalite

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Fig. 2 a General geology of the Viholanniemi volcanic suite. The numbers (1–5) on the map refer to the REE samples collected during this study. bDetailed geology of the Korpela succession. The Korpela area is defined by the current drilling by GTK during 2013–2015

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1.895–1.875 Ga group (e.g. Ekdahl 1993; Lahtinen 1994;Kousa et al. 1994, 2013; Lahtinen et al. 2016; Kousa et al.2018; Mikkola et al. 2018b). The Svecofennian domain main-ly comprises granitoids but also includes a large proportion ofsupracrustal rocks of turbiditic metasedimentary origin, in-cluding metamorphosed black shales and minor metavolcanicinterlayers (Fig. 1b). The regional deformation is characterisedby nappe-style, northerly verging overthrusts and recumbent,isoclinal folds formed during D1-D2 responsible for the devel-opment of the main S2 foliation. The D3 deformation wasmainly active after the peak metamorphism at 1.83–1.81 Ga,overprinting all the metamorphic zones formed during the D2

(Korsmann et al. 1988). The D3 deformation structures are cutby ca. 1.8 Ga post-tectonic granites (Vaasjoki and Sakko1988) and partly folded by D4 deformation (Kilpeläinen1988). The development of NE-trending crustal-scaletranstensional shear zones and faults during D3-D4 resultedin the juxtaposition of crustal blocks with discordant meta-morphic zones (Korsman et al. 1984; Hölttä 1988).

The Jäppilä-Virtasalmi block (Fig. 1c) is part of the south-eastern extension of the older, 1.93–1.91 Ga, primitive arccomplex of rocks of the Northern Ostrobothnia supergroupforming three major suites (Fig. 2a): the Maavesi, Virtasalmiand Viholanniemi suites (Kousa et al. 2018). A major part ofthe Jäppilä-Virtasalmi block consists of schists and gneisseswhich are turbiditic in origin and are intruded by gabbro,granite, granodiorite and tonalite of the Maavesi suite. TheVirtasalmi suite is mostly dominated by submarine sub-alkaline medium-K tholeiitic basalts and andesites (Huhma1986; Vaasjoki and Sakko 1988; Lawrie 1992; Korsmanet al. 1997; Pekkarinen 2002; Hokka and Virnes 2018;Kousa et al. 2018). The basalts have an E-MORB or within-plate basalt affinity and have been interpreted to indicate therifting of mature arc complexes (Lawrie 1992). TheViholanniemi suite is characterised by sub-alkaline, subma-rine, bimodal volcanic-sedimentary formations surroundedby mica gneisses and mica schists (Fig. 2a). TheViholanniemi suite includes rhyolitic quartz feldspar porphy-ries, intermediate pyroclastic rocks and lavas with mafic inter-flows or intercalations, and minor carbonate rock interlayers(Korsman 1973; Zhang 2000; Kousa 2009). The southern partof the Viholanniemi area comprises more mafic-dominatedsequences, locally pillowed lavas that are classified into theVirtasalmi suite (Kousa et al. 2018; Zhang 2000). Instratigraphically upward progression, the mafic volcanic rocksoverlie the felsic sequences. The regional metamorphism wasstudied by Korsman et al. (1984, 1988) which resulted in theclassification of progressive metamorphic zones towards theSE from Viholanniemi. Two episodes of regional metamor-phism have been recognised in the Viholanniemi region: aprograde lower and middle amphibolite facies episode associ-ated with the main deformation event D2 (Korsman et al.1984, 1988).

Synvolcanic intrusions of the Viholanniemi suite

The Saunakangas intrusion is a multiphase tonalitic granitoidforming the main lithodeme of the Maavesi suite, covering anarea of 50–60 km2 (ESM Fig. 1). It has a pinkish-white colourand textures from porphyritic to granoblastic and locally it isstrongly foliated and sheared. Miarolitic cavities are locallyabundant and occasionally filled with epidote and quartz.Typically, plagioclase (albite) is altered to saussurite andsericite and includes epidote (Fig. 3a). Locally, near the intru-sion boundary, it contains abundant xenoliths of mafic volca-nic rocks (Fig. 3b). Approximately 1 km north of Korpela, thetonalite is transected by fine-grained aplite dykes (Fig. 3c).The typical alteration feature in the Saunakangas intrusion isepidotisation which form patches of over 80 mm in diameter(Fig. 3a). The pinkish colour is caused by K-feldspar andsericite alteration that is mainly concentrated near the shearzones (Fig. 3d). In the vicinity of volcanic rocks, the intrusioncontains minor amounts of finely disseminated magnetitewhich can also be detected in magnetic surveys as positiveanomalies (unpublished GTK data). The U-Pb zircon agefrom two granodiorite-tonalite samples (ESM Fig. 1) yieldedages of 1908 ± 2 Ma and 1912 ± 3 Ma (Kousa et al. 2018).This is closely coeval to the age of Viholanniemi rhyolite,1914 ± 3 Ma (Kousa et al. 2018) indicating the synvolcanicnature of the intrusion. The Saunakangas intrusion shares fea-tures, such as epidote- and quartz-filled cavities, with theKokkokangas granodiorite of the Pyhäsalmi area which is alsointerpreted as a subvolcanic intrusion associated with thePyhäsalmi VMS deposit (Ohtomaa 2014).

Quartz-feldspar porphyry dykes intrude the volcanic andvolcaniclastic rocks at Korpela (Fig. 3d, f). The dykes aremainly concordant to semi concordant and range in thicknessfrom 0.5 to 5 m. The porphyries can be subdivided into twosubgroups: felsic quartz-eye and dacite types. The felsicquartz-eye porphyries are typically reddish, homogeneousand massive. The K-feldspar, plagioclase and quartz pheno-crysts range from 2 to 4 mm in size; they are mainly subhedraland occur in a fine-grained form in groundmass (Fig. 3f). Thedyke contacts are typically sharp and lobate suggestingthat they were intruded into an unconsolidatedvolcaniclastic strata. The dacitic porphyries have a moremafic groundmass and distinct reddish phenocrysts of2–4 mm in size that have partly altered to muscovite(Fig. 3g). Some chlorite and carbonate are also present.The porphyries have been affected by varying degreesof synvolcanic intrusion–related K-feldspar, sericite andsaussurite hydrothermal alteration (Fig. 3d, g).

Local geology at Korpela

The Korpela occurrence, within the Korpela succession, islocated in the central portion of the Viholanniemi suite and

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constrained by several NE- and NW-trending, small-scaleductile shear zones. Based on geophysical potential fielddatasets (GTK unpublished data), the shear zone has a NW-trending wedge-like geometry. This is interpreted as a high-strain corridor forming a dextral shear zone and hosting themain Korpela hydrothermal alteration domain that is over1.5 km long (Fig. 2b).

The Korpela succession can be subdivided into five majorunits: andesite, felsic quartz-phyric rhyolite with minor carbon-ate interlayers, intermediate to felsic volcaniclastic rock, basalticsill and synvolcanic intrusives. Andesite sills and flows are in-terbedded with the felsic volcanic and volcaniclastic units.Basaltic sills or dykes and synvolcanic intrusives overlie but alsolocally intrude into the Korpela volcanic succession. Thesynvolcanic quartz-feldspar porphyry dykes (the quartz-eye

Fig. 3 Outcrop and drill corephotographs of the subvolcanicintrusion and synvolcanic felsicdykes at Viholanniemi andKorpela. a Miarolithic cavitiesand epidote alteration in theSaunakangas tonalite atVuoriniemi (sample 5, Fig. 2). bXenoliths of mafic volcanic rocksin the Saunakangas tonalite atVuoriniemi (sample 5, Fig. 2).The length of the pen is 16.5 cm. cA fine-grained aplite dyke cutsthrough the Saunakangas tonalitephase at Vuoriniemi (sample 5,Fig. 2). d Locally granitised,high-K-feldspar, rock at thecontact zone of andesite flows andthe Saunakangas tonalite. e Lightcoloured, fine-grained quartz-phyric rhyolite. f Quartz-feldsparporphyry (‘quartz-eye porphyry’)dykes with an intense red colour.These have a chemicalcomposition similar to the graniticrocks close to the Saunakangastonalite. g Dacitic feldspar-quartzporphyry (dacite porphyry) dykeat Korpela. This rock is at thecontact with quartz-feldsparporphyry. Ep epidote

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porphyries) are homogeneous, massive rocks with sharp con-tacts with the surrounding volcanic and volcaniclastic rocks.They are similar to the quartz-phyric to aphyric rhyolites thatform the spatially large, coherent, felsic volcanic package westof Korpela (Figs. 2a, b and 3e). Volcaniclastic rocks are mainlycomposed of pyroclastic volcanic fragments that are generatedfrom explosive volcanic activity. These rocks include pyroclas-tic breccia (see Fig. 5g), lapilli tuff and tuffite. Locally,autoclastic textures can be seen with jigsaw-fit texture indicative

of in situ (carapace) breccia. The volcaniclastic fragments arefelsic to intermediate in composition, mainly monomict, poorlysorted and clast- to matrix-supported. Strong deformation andpenetrative S2 foliation have partly destroyed the primary tex-tures, and fragments and clasts are now elongated along thestretching lineation. West of the Korpela succession, narrowinterlayers of carbonate rocks were observed during drilling inthe felsic volcanic units. A polymictic clast-supportedvolcaniclastic conglomerate with felsic, intermediate and mafic

Fig. 4 a Andesitic volcanicfragments within the felsicvolcanic unit (Outcropobservation: 6888056 N; 539915E). The hammer handle is 80 cmlong. b Transposed folding ofmafic lava in rhyolite unit(Outcrop observation: 6887993N; 540003 E). c Deformationfabrics in pervasively sericitealtered rock with andalusiteporphyroblasts in drill core(DH24, 218.20 m). Andalusiteporphyroblasts are orientedparallel to the main foliation. dCrenulation observed in drill core(DH10, 83.70 m). e Pervasivelyaltered and deformed andalusite-sericite schist (DH17, 128.30 m).And andalusite, C crenulation, Ffoliation, Ser sericite

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volcanic pebbles and rare mica schist and granitoid pebbles arepredominant in the east of Korpela (Fig. 2b) (Kousa 2009).

A large part of the rocks located in the Korpela area havebeen affected by intense alteration and all the rocks are affect-ed by polyphase (D1-D4) deformation and metamorphismresulting in the near-complete destruction of the primary vol-canic textures. Thus, it is difficult to identify the primary

volcanic textures. At Korpela, the rocks are deformed byisoclinal F2–3 folding relating to the regional D2–3 stage(Kilpeläinen 1988). The D2 is characterised by intense ductiledeformation which can be observed by complex fold interfer-ence patterns in outcrops where F2 folds are refolded by F3folds (Fig. 4a, b). The deformation during D4 is brittle-ductilewith pegmatites and sulphide-bearing quartz veins favouring

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NW to NNW striking D4 fractures and conjugate sets. The D4

stage brittle faults cut the lithological units into separate tec-tonic subdomains. The chemostratigraphy and alteration map-ping together with interpreted lithological contacts and therepetition of the main rock units support the folded and faultedpattern in the Korpela succession. Volcanic rocks in the shearzone have a strong foliation and stretching lineation with localcrenulation. The strong penetrative NW-trending S2 foliationand associatedmineral and stretching lineation L2 affects mostof the silicate minerals. The main axial plane foliation of S2dips 60–80 degrees towards the SW. According to regionalobservations by Kilpeläinen (1988), the D3–4 deformation isusually shown as zonal crenulation (S3–4) deforming of S2foliation. Local crenulation of S3–4 has preferentially beendeveloped in thin bands of sericite which also can be clearlyobserved in the Korpela drill core (Fig. 4c–e).

Viholanniemi Zn-Cu-Pb-Ag-Au deposit

The Viholanniemi Zn-Cu-Pb-Ag-Au deposit (Figs. 1c and 2a)has been reported as a historic, non-compliant, inferred re-source of 250,000 t at 2.1% Zn, 0.2% Cu, 0.64% Pb,65 ppm Ag and 0.9 ppm Au (Västi 2012). The Viholanniemideposit has not been mined and it can be classified as a small,early-stage, exploration target. It is situated within the north-ern part of the Viholanniemi suite (Makkonen 1991; Zhang2000). It was discovered by GTK in 1985 by combining in-formation from glacial erratic boulder tracing, till geochemis-try and airborne and ground geophysical surveys. The sul-phides are predominantly hosted by quartz-carbonate-(± trem-olite) gangue which forms conformable veins and, locally, as

stockwork of veinlets in felsic to intermediate volcaniclasticand felsic quartz porphyry rocks. The average thickness ofindividual veins is approximately 1–5 cm (Makkonen 1991;Zhang 2000). The ore minerals are sphalerite, galena, chalco-pyrite, pyrite and pyrrhotite. Gold and silver are present asvarious alloys. Gold is mainly in electrum as fine-grainedinterstitial grains between silicates and carbonates(Makkonen 1991). The Viholanniemi ore body is associatedwith a distinct, pyrite-sericite dominant, alteration assemblagewith local epidotisation. According to Zhang (2000), thesericite-quartz altered rocks show a minor Na2O depletion.

The mineralised zone is approximately 600 m long com-prising two separate lenses with an average thickness of 1.1 m.Makkonen (1991) interprets the mineralised lenses to bestructurally controlled by the F3 folding that may representsmall folds in limbs of a larger asymmetrical fold structure.The primary axial plane of the F3 folds trends northwest-southeast and the fold axis plunges to the southeast(Kilpeläinen 1988; Makkonen 1991). The Viholanniemi oc-currence is also affected by the late D4 stage brittle deforma-tion that cuts the lithological units by faults into separate parts(Makkonen 1991).

Mineralisation

In 2013, GTK discovered a mineralised biotite-chlorite-garnet-anthophyllite boulder with 0.9 wt% Cu next to a gravelroad at Korpela (Figs. 2b and 5a and ESM Fig. 2c). Thisresulted in follow-up work including a till geochemical sur-vey, ground geophysical surveys, geological mapping and atotal of 3300 m of diamond drilling (Hokka et al. 2014) (Fig.2).

The style of sulphide mineralisation varies from semi-massive to stringer zones and dissemination (Fig. 5g–l).Drill core samples show a clear base and precious metalsenrichment which makes it an attractive exploration targetwhere an economically viable massive sulphide deposit isyet to be discovered. The main sulphide minerals are pyrite,pyrrhotite with minor chalcopyrite and sphalerite. Magnetiteand ilmenite represent the oxide minerals. Semi-massive sul-phides are typically pyrite with minor pyrrhotite. Sulphidespredominate in interstices and open spaces or replace unstablegangueminerals. Locally sulphides, mainly pyrite, have partlyor fully replaced lapilli tuff fragments. The sulphide stringersconsist primarily of pyrite and minor chalcopyrite and areeither interstitial between clasts of volcaniclastic rocks or ori-ented parallel to the lineation (Fig. 5g, i). Locally, pyrite vein-lets are tightly folded and transposed following the crenulationbut also as infill of fractures cutting the main foliation (ESMFig. 2a (2), ESM 2b (1,2)). Native gold has only been ob-served under the microscope. The amounts of sulphides dis-play a positive correlation with the alteration intensity. Thesulphide-bearing fractures subparallel to S2 together with an

�Fig. 5 Drill core photographs of the main chemical alterationassemblages and styles of sulphide mineralisation observed at Korpela.a Biotite-chlorite-garnet-anthophyllite altered andesite A1 whichrepresents the Mg-Fe-S alteration type. Locally containing stringerchalcopyrite-pyrite mineralisation. b Muscovite-biotite-andalusite-garnet (± staurolite and talc) schist which presents the K-Al-Fe-(± S)type. Typically containing pyrite dissemination and stringers. cPervasively altered, white, sericite-muscovite schist which representsthe K alteration type. d Aluminium-rich mineral assemblages(andalusite, staurolite ± sillimanite and cordierite) which represent theK-Al-Mg-Fe-S alteration type. Mainly pyrite and pyrrhotite as semi-massive and stringer zones. e Brecciated quartz-carbonate-muscovite-pyrite representing the Si-K-Ca-(± S) alteration type. f Foliatedstaurolite-biotite-sericite schist. g Pyrite dissemination in stringers ofpyrite in between weakly quartz-, sericite- and chlorite-altered rhyolitelapilli. h Folded and transposed pyrite veinlets within a muscovite-sericite-biotite rich matrix. i Pyrite blebs and pyrite-pyrrhotite stringersin variable chlorite, quartz, muscovite-altered schist. j Pyrite stringers inmuscovite-sericite rich rock. k Schistose and crenulated, stronglyaluminous, altered rock with transposed and crenulated pyrite stringers.Locally, the andalusite porphyroblasts are massive. l Pyrrhotitedissemination with minor pyrite in sericite-quartz altered rock. Andandalusite, Ath anthophyllite, Bt biotite, Cal calcite, Chl chlorite, Cpychalcopyrite, Crd cordierite, Fsp feldspar, Grt garnet, Ms muscovite,Po pyrrhotite, Py pyrite, Qtz quartz, Ser sericite, St staurolite

Miner Deposita

increase in sulphide grain size indicate the remobilisation ofsulphides during metamorphism.

Alteration

Alteration of variable intensity defines steeply dipping pipe-like bodies in the Korpela volcanic succession. The alterationat Korpela is mainly characterised by metamorphosed alumi-nous, sericitic, chloritic and albite alteration. Based on thecurrent drilling, discordant and laterally restricted intense al-teration forms a N to NW-trending domain that has a strikelength of at least 1.5 km (Fig. 2b). The altered zone is locatedpredominantly within the volcaniclastic facies rocks, boundedby the porphyritic rhyolite unit to the east (Fig. 2b). The min-eral assemblages produced by the hydrothermal alteration areoverprinted by metamorphic mineral assemblages that wereformed during the D2 progressive stage which peaked at mid-dle amphibolite facies (Kilpeläinen 1988). Alteration is

mainly characterised by sericitic, aluminous and chloritic al-teration consisting of the following diagnostic minerals invarying amounts: quartz, muscovite, biotite, sericite, andalu-site, chlorite, garnet, chlorite, carbonate, staurolite, silli-manite, cordierite, accessory rutile and tourmaline(Figs. 5a–f and 6a–f). Certain aluminous minerals, suchas andalusite, occurring as large porphyroblasts (≤ 5 cm)along the main foliation, are re-oriented parallel to, orovergrow, the main S2 foliation (Fig. 5k). It is generallynot possible to see through the metamorphic overprintbut occasionally volcanic textures are visible in theweakly altered parts. The alteration at Korpela is classi-fied according to mineral assemblages and chemical al-teration types based on observations of key mineralabundance, textures and chemical whole-rock assay data. Atotal of six chemical alteration types can be defined: Mg-Fe-S,K-Al-Fe-(± S), K-Al-Mg-Fe-S, K, Si-K-Ca-(± S) and Ca-(±Na) (Fig. 5a–f), as described in detail below.

Fig. 6 Thin sectionphotomicrographs showingfeatures of hydrothermally alteredvolcanic rocks from Korpela. aStrongly sheared and pervasivelyaltered sericite-quartz-andalusite-cordierite schist (K-Al-Mg-Fe-Salteration type) (DH17, 147.40 m,XPL). Andalusite and cordieriteporphyroblasts with pyrite grainsin muscovite and sericite matrix. bBiotite-chlorite-garnet-anthophyllite mineral assemblage(Mg-Fe-S alteration type) withpyrite and chalcopyrite fromKorpela (Bt-Chl-Grt-Ath schist,Cu-rich outcrop, PPL). cCarbonaceous siltstone (DH19,106.90 m, XPL). Layeredcarbonate rock; with quartz-phlogopite-musovite assemblagewith pyrite dissemination (Ca-(±Na) alteration type). d Quartz-eye porphyry; showing euhedraltwinned plagioclase with weaksericite alteration and quartzphenocrysts (DH21, 48.00 m,XPL). e Asymmetric pressureshadow around a garnetporphyroblast (DH7, 111.10 m,PPL). f Pervasively sericite alteredand intensely deformed sericiteschist. The shear band indicatesdextral movement (DH7,111.10 m, XPL). And andalusite,Bt biotite, Cal calcite, Chl chlorite,Crd cordierite, Grt garnet, Msmuscovite, Pl plagioclase, Sersericite, XPL cross polarized light,PPL plane polarised light

Miner Deposita

Methods

The study is part of GTK’s Regional Ore Potential MappingProject and was conducted intermittently during the period2013–2016 and comprised field observations, surface and drillcore sampling, petrography and whole-rock geochemistry.

For this study, a total of 230 drill core samples from 18diamond drill holes (3313 m drilled) were collected to recon-struct a representative dataset for the main volcanic units ofthe Korpela succession (Fig. 2). In addition, five outcrop grabsamples were collected from different intrusive phases of theSaunakangas intrusion (Fig. 2a). Prior to sampling, geologicalmapping and core logging were executed and systematicallydocumented in respect to mineralogy, textures, structures,contacts and alteration intensity. The typical length of an

individual half core drill sample taken was 40 cm. The corediameter was 41.7 mm. The sampling interval was based ongeological logging observations ensuring that the representa-tive proportion of both least-altered rocks and main alterationassemblages were included.

All the samples were analysed during the period 2013–2016 by Labtium Oy, Rovaniemi, Finland. Samples weredried at 70 °C and the standard scheme of crushing consistedof direct one-stage fine crushing using a special type jawcrushers and precision riffle splitting. The pulverizing (tonominal > 90% < 100 μm) was done using a low-chromebowl with quartzite cleaning done after every sample.Analyses were done using the wavelength dispersive X-rayfluorescence technique (WD-XRF) from pressed powder pel-lets. Carbon was analysed from all samples by Leco. The REE

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Fig. 7 Immobile-element ratio plots for the Korpela target. Each rocktype forms its own co-genetic group which are highlighted by theellipsoids. The dashed lines represent fractionation trends and crossesare samples that represent the precursor rock type of each group. a Zr/TiO2 vs Al2O3/TiO2 plot of the chemical groups showing a continuoustrend from basalt to rhyolite. Some groups show more scatter which maybe caused by the sedimentary enrichment of feldspars in clastic rocks orreflect mixed provenance (e.g. dacite and rhyodacite). b Zr/Al2O3 vs Zr/

TiO2 plot. c Zr/Al2O3 vs Al2O3/TiO2 plot. d Zr/Yvs Zr/TiO2. eAl2O3-Zr(compatible-incompatible) bivariate plot showing a slightly negativefractionation trend with polynomial fit (y = − 3E-05 × 2 + 0.004x +16.777, R2 = 0.673). f TiO2 vs. Zr (compatible-incompatible) plotshowing a negative polynomial fractionation trend (y = − 7E-06 × 2 +0.0011x + 1.0598, R2 = 0.97) typical for co-genetic calc-alkalinevolcanic suite

Miner Deposita

and refractory trace elements were assayed by inductivelyplasma mass spectrometry (ICP-MS) following lithium boratefusion and acid digestion. REE assays were mainly done onthe grab samples from the Saunakangas intrusion. Internalquality control (QC) procedures included blanks, replicatesand standards which were used to control the quality ofwhole-rock XRF and ICP-MS analyses.

Due to the pervasive alteration at Korpela, the only reliablemethod of determining the precursor volcanic rock type is touse the immobile element method. The results of chemicalclassification for Korpela rock samples are described inESM Tables 1 and 2. First, the element mobility was testedusing data from altered and unaltered samples against the po-tentially immobile high-field-strength elements (HFSE) fromeach visibly uniform volcanic unit. The test for immobilitywas conducted for Al, Ti, Zr, Nb and Y to see highly corre-lated linear trends for a single-precursor system (Barrett andMacLean 1991; McLean and Barrett 1993). These elements

are potentially immobile with sufficient abundance (i.e. con-centrations not too close to the detection limits of the assays)to ensure accurate chemical analyses. All the rock units includ-ed samples from variably altered and unaltered rock. The Zr-TiO2 ratio was chosen as the most reliable immobile elementmonitor because it was found that it provided the best magmat-ic fractionation curve as defined from least-altered samples ofeach of the chemical rock types (Fig. 7f). Samples were thengrouped according to chemical rock types based on differentco-genetic linear trends and immobile-element ratios. Thegroups were validated with other immobile-element plots tosee if any overlap occurred (Fig. 7). Samples that did not fitinto any of the immobile-element plots tested were treated asoutliers. Samples were then further subdivided into magmaticaffinity groups based on their Zr/Y ratios. For each chemicalrock type, a least-altered sample was selected as an approxi-mation of the precursor composition to determine the primaryfractionation trend. The precursor samples were selected on the

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sulphide-rich parts can be seen as elevated S and Fe contents. Themineralised zones can be observed from rhyolite to andesite and no keystratigraphic horizon has yet been distinguished. The alteration types andstyle of mineralisation are described in Fig. 5

Miner Deposita

basis of (1) having no visible secondary minerals, (2) an ab-sence of visible sulphide minerals (< 1 wt% S) and (3) follow-ing a normal major element composition. The least-alteredsamples were then checked in order to define reasonable frac-tionation trends and best-fit curves for different immobile ele-ments against Zr. More details of the procedure and a fullerdiscussion of the results of the mass balance calculations isprovided in Hokka (2020).

Results

Immobile-element geochemistry

Immobile element geochemistry was used to reconstruct thevolcanic stratigraphy for the intensely altered, metamorphosedand deformed rocks. A total of 12 main chemostratigraphicunits and 21 chemical rock types, ranging from basalt to rhy-olite, were defined based on different co-genetic linear trendsand Al2O3/TiO2, Zr/Al2O3, Zr/TiO2, Zr/Y and Zr/Nb ratios(Fig. 7a–d, ESM Table 2). The least-altered precursor samplespresented in ESM Table 3 were used to define the primary

magmatic fractionation trends (Fig. 7e, f) fromwhich the massbalance could be determined for the altered rocks (Hokka2020). The least-altered samples form a well-defined fraction-ation line that was fitted using a second-order polynomialcurve in Zr/TiO2 immobile element plot. The polynomial frac-tionation curve shows Ti depletion and Zr enrichment whenmoving from basalt to rhyolite composition (Fig. 7f).

The effects of net mass changes can be seen by de-viation of samples from the fractionation trend in theZr/Al2O3 and Zr/TiO2 plots (Fig. 7e, f). Therefore, therock types were validated by immobile element ratios inscatter plots to remove the effect of hydrothermal alter-ation (Fig. 7). The groups were further subdivided basedon their magmatic affinity given by the values of theZr/Y ratio (Fig. 10b; ESM Table 2) and the Zr/Y vs Zr/TiO2 scatter plot (Fig. 7d). This yielded a total of 12chemostratigraphic units and 21 chemical rock types(ESM Table 2).

The volcanic rocks range from basalt to rhyolite, comprisingeight chemostratigraphic groups and 17 chemical rock types(ESM Table 1). Rhyolites are the dominant chemostratigraphicunit forming three distinct subgroups which are further

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intense alteration (shaded) is drawn using a combination of implicit 3Dmodelling (Hokka 2020) and logging information. The rock typediscrimination in Fig. 9b is based on lithogeochemical classification andthus differ from rock types presented in Fig. 9a due to the intensivealteration, deformation and metamorphism which disabled thedetermination of protoliths by visual inspection

Miner Deposita

subdivided into 6 different chemical rhyolite rock types all hav-ing a calc-alkaline affinity (Fig. 10b). The textures of felsic rocksvary from clearly porphyritic to volcaniclastic and aphyric. Therhyolite A is described as having coherent volcanic textureswhereas rhyolite B and C are clearly volcaniclastic rocks. Therhyolites B and C are indistinguishable without lithochemical

data. Rhyodacites and dacites are predominantly volcaniclasticand have both calc-alkaline and transitional magmatic affinities.Andesites have been grouped in two (A and B) and furthersubdivided into five subgroups based on their magmatic affinity.Andesites A and B can be distinguished by the Zr/Al2O3 and Zr/TiO2 immobile ratios and based on their spatial distribution. In

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Miner Deposita

the field, andesite A is characterised by coherent, aphyric tofeldspar-phyric textures and abundant quartz-carbonate veining.Andesite A is transitional between calc-alkaline and tholeiitic.Andesite B is predominantly volcaniclastic, less coherent in tex-ture and predominantly calc-alkaline with a minor transitionalaffinity. The mafic rocks are all tholeiitic basalts. Synvolcanicfelsic porphyry dykes and tonalities are treated separately and donot form part of the volcanic chemostratigraphy due to the ab-normal Al2O3/TiO2 and Zr/Al2O3, element ratios (ESMTable 2). The iron sulphides are mainly associated with daciteA1, rhyolite B1 and andesite B (Figs. 8, 9 and ESM Fig. 2a, b),whereas the copper sulphides are mainly concentrated in andes-ite A1 (Fig. 5a, ESM Fig. 2c).

The Korpela area is characterised by sub-alkaline bimodalvolcanic succession (Fig. 10a). Rhyolites are mainly calc-alkaline with elevated HFSE contents (Zr > 200 ppm) andcan be classified to FIIIa to FII rhyolites of Lesher et al.(1986) (Fig. 10b–f). In the Nb-Y plot, the felsic rocks have atrend towards within-plate (A-type) magmatic affinities(Fig. 10e). The basaltic sills or dykes that cross-cut the alter-ation domain at Korpela and the basalts that overlie theViholanniemi suite are all of a tholeiitic MORB/BABB typecharacterised by elevated HFSE contents with (e.g. TiO2 andNb) (Fig. 10d). The stratigraphy of the Viholanniemi suiterelated to the Korpela succession is described in ESM Fig. 3.

Most of the tonalite samples from the Saunakangas intru-sion show steep chondrite-normalised REE patterns withsmall negative to non-existent Eu anomalies (ESM Fig. 4).Sample 5 north of Korpela (Figs. 2a and 3a) differs from therest of the Saunakangas samples as it displays a larger nega-tive Eu anomaly and a steeper chondrite-normalised patternfor the LREE (ESM Fig. 4). The Saunakangas tonalite sam-ples are similar to rhyolites A and B with closely related co-genetic Zr/TiO2 ratios (ESM Table 2). The composition ismainly transitional between calc-alkaline and tholeiitic. The

synvolcanic quartz-feldspar porphyry that cuts across theKorpela succession and quartz-phyric rhyolites, forming thespatially large felsic volcanic package, has very high Al2O3/TiO2 and low Zr/Al2O3 ratios compared to other felsic rocks inthe area (ESM Table 2).

Alteration types

Alteration at Korpela is classified according to mineral assem-blages and chemical alteration types (Figs. 8, 11a and 12). Atotal of six chemical alteration types can be defined after al-teration mineralogy and their chemical identifiers: Mg-Fe-S,K-Al-Fe-(± S), K-Al-Mg-Fe-S, K, Si-K-Ca-(± S) and Ca-(±Na) (ESMTable 4). The alteration types form zonation aroundthe sulphide-rich parts and extend horizontally for at least1 km (Fig. 11b). The footwall alteration at Korpela is mostlyintense and pervasive resulting in total or near total destructionof the primary textures.

Mg-Fe-S alteration

The Mg-Fe-S alteration is defined by biotite-anthophyllite-garnet, biotite-chlorite-garnet-anthophyllite, biotite-sericite-chlorite-anthophyllite and carbonate-biotite-tremolite-anthophyllite-chlorite mineral assemblages (Figs 5a and 6b).The anthophyllite and garnet occur as porphyroblasts alongthe main foliation (ESM Fig. 2c). Besides orthoamphiboles,the Mg-Fe-S rocks contain accessory chlorite, tremolite,cummingtonite and carbonate. There is no muscovite or alu-minosilicates present within this alteration type. The Mg-Fe-Salteration is associated with andesite A, which is predominant-ly located in the northern part of the Korpela succession (Fig.2b). The alteration type contains mainly chalcopyrite-pyritestringers. In chemostratigraphic ternary diagrams, the Mg-Fe-S altered samples plot towards the cordierite, garnet andpropylitic mineral nodes following the chloritic alterationtrend (Fig. 12).

K-Al-Fe-(± S) alteration

The K-Al-Fe-(± S) alteration is a muscovite-sericite-bearingrock with abundant biotite and aluminosilicates (andalusite ±staurolite) (Fig. 5b). The diagnostic feature of K-Al-Fe-(± S)alteration is the lath-shaped porphyroblasts of biotite that bothovergrow and cut across the foliation. This alteration type isdefined by sericite-biotite-garnet-andalusite, muscovite-biotite-andalusite-garnet-staurolite and quartz-biotite-sericite-andalusite-talc mineral assemblages. Only minor feldspar andferromagnesian minerals are present. The alteration type con-tains minor pyrite-pyrrhotite stringers. This alteration type oc-curs from rhyolite and dacite to andesite. In chemostratigraphicternary diagrams, the K-Al-Fe-(± S) altered samples plot to-wards the kyanite and sillimanite mineral nodes (Al-rich end)

�Fig. 10 Petrochemical affinity of the least-altered volcanic units in theKorpela succession based on rocks sampled from drill core. a Zr/TiO2 vsNb/Y trace-element discrimination diagram after Pearce (1996). bYvs Zrplot provides the estimate for magmatic affinity ranging from tholeiitic tocalc-alkaline. The discrimination boundaries of the Zr/Yvalues are basedon MacLean and Barrett (1993). c Zr/Y vs Y diagram after Lesher et al.(1986), where most of the felsic rocks (dashed line) have FIIIa to FIIaffinity. The discrimination boundaries of the Zr/Y values are based onMacLean and Barrett (1993) dV vs. Ti/1000 diagram (Shervais 1982) forKorpela mafic rocks. Arc-related basalt (ARC); back-arc basin basalt(BABB); boninite (BON); mid-ocean ridge basalts (MORB); island-arctholeiite (IAT); low-Ti tholeiite (Low-Ti); ocean-floor basalt (OFB);Ocean Island and alkali basalts (OIB). e Nb vs Y plot showing within-plate to A-type rhyolite signatures. f Zr vs Nb plot shows that the Korpelarhyolites are HFSE-enriched and support the evolved arc setting afterPiercey (2007). Diagrams (e) and (f) show the fields of VMS-barren(brown shaded colour) and VMS-hosting rhyolites (blue shaded colour)in post-Archean continental crust-associated setting. The shaded fieldsare after Piercey (2007). The least-altered precursor samples arepresented in ESM Table 3

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which portray the argillic to advanced argillic alteration trend(Fig. 12).

K-Al-Mg-Fe-S alteration

TheK-Al-Mg-Fe-S alteration is characterised bymuscovite withmedium- to coarse-grained aluminosilicates along with stauro-lite and garnet (5–30 vol%). Locally, andalusite porphyroblastsform very large grains (> 5 cm) towards the boundary of thealteration zone. This is observed in DH24 (ESM Fig. 5a).Aluminous nodules are also locally present (ESM Fig. 5b).Theferromagnesian minerals (Mg-Fe amphiboles, chlorite,cordierite) are more abundant (1–5 vol%) compared to the K-Al-Fe-(± S) alteration type. Feldspar is generally minor toabsent. The K-Al-Mg-Fe-S alteration type is defined bysericite-quartz-andalusite-cordierite (Fig. 6a), sericite-andalusite-cordierite, magnetite-biotite-quartz-andalusite,sericite-cordierite-quartz, biotite-garnet-cordierite-staurolite andchlorite-staurolite assemblages. The alteration assemblagecontains accessory sillimanite, rutile and tourmaline. Thealteration type is also more sulphide-rich than K-Al-Fe-(± S)alteration, having abundant pyrite-pyrrhotite stringers and dis-seminated to semi-massivemagnetite which are oriented parallelto the lineation (S2). The alteration is characterised as strong to

pervasive in nature. It characterises rock types from rhyolite toandesite composition and is commonly spatially associated withthe K-Al-Fe-(± S) alteration. In chemostratigraphic ternarydiagrams, the K-Al-Mg-Fe-S altered samples form twosubgroups: andesite host and dacite to rhyodacite host.The andesitic rocks display a slight Al enrichment,whereas the dacitic to rhyodacititc rocks are mainlyconcentrated at the Al-rich end (A, A′) (Fig. 12).

K alteration

The K alteration type consist of muscovite- and sericite-bearing rocks (> 20 vol%). The alteration type is distinguish-able by its white appearance (Fig. 5c). Abundant quartz andsericite (± biotite) is also present. This alteration type is onlylocally present and common between or adjacent to aluminousalteration assemblages. The K-altered zones are barren or con-tain only minor sulphides.

Si-K-Ca-(± S) alteration

The Si-K-Ca-(± S) alteration type is defined as pervasivelysilica-altered rock with > 50 vol% quartz to less-alteredquartz-, sericite (±muscovite) and carbonate-rich rocks. This

-100 RL

-50 RL

0 RL

50 RL

100 RL81DH

NESW

a’71DH

DH

24

-1 RL50

54

05

00 E

6887400 N

a

a

6887500

a

a’

b

540500250m

50 m

Legend

K-Al-Fe-(±S)

K-Al-Mg-Fe-S

Mg-Fe-S

Si-K-Ca-(±S)

K

Ca-(±Na)

Alteration Mapping

Fig. 11 a Drill hole section 6887400 N, 540500 E showing the alterationtypes derived from the metamorphic mineral assemblages of alteredvolcanic rocks. The sulphide-rich (Py + Po) stringers, shown as redmarkings along the drill hole traces, are mainly concentrated in thedomain of K-Al-Mg-Fe-S alteration. b Simplified geological map withspatial distribution of the different alteration types. The sericite-biotite-

aluminosilicate-pyrite alteration assemblage (K-Al-Fe-(± S), K-Al-Mg-Fe-S) dominates in the SW and biotite-anthophyllite-garnet-(± chlorite)alteration assemblage (Mg-Fe-S) in the NE part. The carbonate + albiteassemblage (Ca-(± Na) is interpreted to represent moderate to weakhanging wall

Miner Deposita

includes at least the following assemblages: quartz-sericite-pyrite, quartz-magnetite-fuchsite-pyrite and quartz-carbonate.The Si-K-Ca-(± S) alteration type (Fig. 5e) is associated withan andesite to rhyolite precursor and present as local zones,adjacent to aluminous K-Al-Mg-Fe-S and K-Al-Fe-(± S) andCa-(± Na) alteration types.

Ca-(± Na) alteration

The Ca-(± Na) alteration type includes the Ca assemblagesdefined by carbonate, quartz-carbonate-muscovite but also a‘chaotic’ carbonate assemblage including the carbonate-phlogopite-quartz-chlorite-pyrite assemblage (Fig. 6c). Theseassemblages are spatially distinguishable. Albitisation is alsoincluded in the Ca-(± Na) alteration type, which is character-ized by carbonate, and displays as a moderate to strong pink-white coloured coating.

Discussion

Identifying prospective belts hosting VMS deposits and beingable to further delineate the targets by studying the volcanic stra-tigraphy and alteration is proven to be effective. Generally ingreenfield exploration, lithochemistry of bimodal volcanic as-semblages is used to target rift successions and delineating thepotential VMSdistricts (Gibson et al. 2007; Piercey 2007, 2011).Several critical characteristics exist in regard to volcanic rocks,especially on felsic suites, that help to differentiate potentiallyVMS-prospective and VMS-barren systems (e.g. Lesher et al.1986; Lentz 1998; Barrett and MacLean 1999; Piercey et al.2001; Piercey 2011). Petrochemical evidence has proven thatfelsic volcanic rocks with elevated HFSE and REE contentsand FIII to FII rhyolite composition are commonly associatedwith VMS deposits in post-Archean evolved environments(Lesher et al. 1986; Hart et al. 2004; Piercey 2011).

Crd

Bt

KfsKy,Sil

Ms

Alm,

Prp

Ath,Opx

Sericitic

Potassic

K

Argillic

Advanced

argillic

Fe V

Advanced argillic

Argillic

Sericitic

Chloritic

Fe-alteration

Ky,Sil

Bt

Crd

Opx

Grt

Fe

K

Mg

A’ K

F

Advanced

argillic

Argillic

Ca

Ca-Fe

Fe/Mg

Crd

Ky,Sil

An

Ep

Grs

Cal

Chl Ath

Opx

Tr,ActDi-Hd

Hbl

Alm, Prp

V

A’

C F

A

F M

AFM A’CF

A’KF

Legend

K-Al-Fe-(±S)

K-Al-Mg-Fe-S

Mg-Fe-S

Andesite

Dacite/Rhyodacite

Rhyolite

V

Least-altered

volcanic rocks

a b

c

V

V

Chloritic

Mineral node

Mineral range

Fig. 12 Chemographic diagrams showing the alteration trends for K-Al-Fe-(± S), K-Al-Mg-Fe-S and Mg-Fe-S alteration types at Korpela. Thedata are recalculated to molecular ratios (i.e. chemical assay results arerecalculated by dividing the weight percentage of each oxide by themolecular weight of that oxide). The chemographic diagrams modifiedafter Bonnet and Corriveau (2007); Corriveau and Spry (2014). Thealtered felsic and intermediate volcanic rocks show two trends: (1) thealuminium-enriched volcanic rocks of K-Al-Fe-(± S), K-Al-Mg-Fe-Salteration types following the advanced argillic alteration trend, and (2)the Mg-Fe-S alteration type described as chloritic alteration. The areas of

least-altered volcanic rocks are shown as a grey circle. A′CF: A′ =Al2O3

+ Fe2O3–(K2O + Na2O), C = CaO, F = FeO + MnO + MgO. A′KF: A′ =Al2O3 + Fe2O3–(K2O + Na2O + CaO), K = K2O, F = FeO + MnO +MgO. AFM: A = Al2O3–K2O; F = FeO, M = MgO. Act actinolite; Almalmandine (garnet); An anorthite; Ath anthophyllite; Bt biotite; Calcalcite; Chl chlorite; Crd, cordierite; Di diopside; Ep epidote; Grsgrossular (garnet); Grt garnet; Hbl hornblende; Hd hedenbergite; Kfs K-feldspar; Ky kyanite; Ms muscovite; Opx orthopyroxene; Prp pyrope(garnet); Sil sillimanite; Tr tremolite

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Chemostratigraphy and hydrothermal alterationat Korpela

In this study, immobile-element lithogeochemistry was used todiscriminate between the different lithological units obliteratedby deformation, metamorphism and hydrothermal alteration.The main lithological groups, determined by immobile ele-ments, range from rhyolite via rhyodacite, dacite and andesiteto basalt. The least-altered samples form a well-defined frac-tionation line indicating a comagmatic volcanic group (Fig. 7f).Thus, the continuum of compositions and having overlappingsamples means that the discrimination becomes locally subjec-tive and samples at the chemical boundaries could representeither of the two rock types. There is also some scatter to beseen in the intermediate volcaniclastic rock units which mightindicate mixed provenance or feldspar enrichment (Fig. 7). InGifkins et al. (2005), the uncertainty related to inhomogeneousvolcaniclastic rocks when determining immobile element ratioswere described as possible compositional variations due to me-chanical sorting of compositionally different clasts during erup-tion and transport, or mixing of debris from different volcanicor external sources in mass flows. The lithogeochemical ap-proach using detailed chemical classification allowed us to de-tect new features of the overall stratigraphy, especially from thestrongly altered parts of the sequence (areas of ‘intense alter-ation’ of Figs. 8 and 9) and where immobile element ratio-based techniques allowed us to classify the precursor rockseven in totally altered rocks.

In VMS and other hydrothermal environments, the meta-morphic mineral assemblages can be characterised and corre-lated with the bulk geochemical compositions and used toidentify the premetamorphic alteration zones; this can be usedin vectoring towards the potential ore (e.g. Dusel-Bacon,2012; Galley et al. 1993; Corriveau and Spry 2014; Caté2016; Dubé et al. 2007a). The presence or absence of distinc-tive coarse-grained minerals reflects not only VMS-style hy-drothermal alteration and P–T conditions during metamor-phism, but also the thermal gradient during metamorphism(Dusel-Bacon, 2012). Hydrothermal alteration at Korpela isclassified into six alteration types. The most extensive alter-ation is the outer margin of the altered hanging-wall zonedefined by weak sericite ± carbonate and locally pervasivealbite alteration (Ca-(± Na)) (Fig. 11a). The footwall alterationis defined by aluminous and sericitic alteration types dominat-ed by sericite-biotite ± andalusite ± garnet ± sericite ± stauro-lite ± talc (K-Al-Fe-(± S)), sericite ± quartz ± andalusite ±cordierite ± staurolite ± garnet ± chlorite (K-Al-Mg-Fe-S)and muscovite-sericite (K) mineral assemblages in mainlyfelsic to intermediate coherent and volcaniclastic rocks. A lessabundant and more restrictive alteration is biotite-anthophyllite-chlorite ± garnet ± sericite ± tremolite ± carbon-ate (Mg-Fe-S) in coherent basaltic andesite (andesite A) sillsand quartz ± sericite ± carbonate ± magnetite assemblage (Si-

K-Ca-(± S)). According to Corriveau and Spry (2014), theamphibolite facies analogues to chlorite-rich footwall (inner)alteration zones of VMS deposits are commonly characterisedby cordierite, orthoamphibole, Al2SiO5 polymorphs (andalu-site, kyanite, sillimanite), garnet or staurolite, quartz and pla-gioclase. In the outer alteration zone or close to high-T alter-ation pipes, aluminous minerals including Al2SiO5 poly-morphs, garnet, chloritoid and staurolite are diagnostic(Dusel-Bacon 2012; Corriveau and Spry 2014). The alumini-um enrichment is interpreted to reflect the leaching of alkalisunder high fluid/rock ratios (Dusel-Bacon 2012). The alter-ation mapping shows a clear zonation of the main alterationtypes at Korpela which can be interpreted to present chloriteand aluminous alteration assemblages (Fig. 11).

The typical chlorite-rich footwall alteration zones, and theirmetamorphosed counterparts of cordierite + anthophyllite as-semblages (Gifkins et al. 2005; Dusel-Bacon 2012; Caté2016), can be interpreted to be present at Korpela by theMg-Fe-S and K-Al-Mg-Fe-S alteration types. The Al-rich al-teration type is not found to be hosted by any particular pre-cursor rock type. This is evident in DH17, where the alterationis dominantly of K-Al-Mg-Fe-S type having the precursor ofandesite B1 to rhyolite A1 (Fig. 8). This is also evident in thechemographic diagrams (Fig. 12). Barrett et al. (2005) dem-onstrated also at the Kristineberg deposit in Skellefte district inSweden that very different metamorphic mineral assemblages(andalusite-quartz-muscovite and cordierite-chlorite-talc) maybe produced from the same felsic precursor rock type. AtLalor Lake (Manitoba, Canada), the high-temperature chlo-rite-dominated stringer zones are located in footwall Mg-Fealteration halo, whereas massive sulphides are constrained bythe proximal K or Mg-Ca alteration zones (Caté 2016). Theapparently large size of the alteration domain (> 1 km) atKorpela is likely related to the occurrence of multiplemineralised intervals at different stratigraphic positions andmay reflect the longevity of the hydrothermal system. Thefavourable ore-hosting unit or bracketing units are interpreted,based on the presence of sulphide stringers and strong hydro-thermal alteration, to be in andesite A1-A2 and B1, dacite A1,rhyodacite A1-A2 and rhyolite A1-B1. The predominant al-teration mineral assemblages at Korpela are sericite-biotite ±quartz ± andalusite ± garnet ± staurolite ± talc (K-Al-Fe-(± S))and sericite-andalusite-cordierite ± garnet ± staurolite ± chlo-rite (K-Al-Mg-Fe-S) and muscovite-sericite (K), which sug-gest lower temperature and acidic conditions. The presence ofaluminosilicates is a diagnostic feature of the metamorphosedequivalent of advanced argillic alteration assemblages whererocks are leached by acidic hydrothermal solutions at highfluid/rock conditions (e.g. Barrett et al. 2005; Dubé et al.2007b; Corriveau and Spry 2014). No signs of boiling or acidaluminium-rich alteration being genetically connected toshallow-water conditions can be shown at Korpela becauseof metamorphism and deformation.

Miner Deposita

The alteration domain at Korpela is not continuous butmore likely form several, discontinuous, parts. This is consid-ered to be due to the overprinting strong deformation and latefaulting. Although alteration types are recognised in severaldrill holes, the connectivity of particular alteration zones inhorizontal and vertical dimensions is uncertain due to thesparse drill holes and lack of outcrop exposures. However,alteration mapping based on the current drilling data enablesus to distinguish zonation between the different alterationtypes (Fig. 11). The hanging-wall sequence is clearly repre-sented by the Ca-(± Na) alteration having a distinct albitealteration and carbonate veining. It is commonly observed thatalteration spatially correlates with deformation zones. It iscommonly the case that the aluminium-rich alteration zonesare intensely sheared to sub-parallelism with the high-strainzones and turned into schist, due to the ductile nature of Al-rich phyllosilicates (Dubé et al. 2007a). Although the intense-ly altered rocks predominate in the Korpela area, weakly andmoderately altered rocks are locally present. This en-ables us to identify the primary rock types and theirtextures which are a prerequisite for the mass balancecalculations in a mineralising system (MacLean andBarrett 1993; Barrett and MacLean 1994).

Comparison between Viholanniemi, Pyhäsalmiand global VMS systems

The Viholanniemi suite belongs to the same, olderSvecofennian magmatic, sequence (> 1.91 Ga) as theVihanti-Pyhäsalmi belt in the RLSZ. Korpela and Pyhäsalmiboth comprise deformed and metamorphosed bimodal-felsic,sub-alkaline, volcanic rocks occurring close to subvolcanicfelsic intrusions.

Nevertheless, Korpela differs from the Pyhäsalmi area inhaving evidence of HFSE-enriched (A-type) rhyolites of calc-alkaline affinity and overlain and cross-cut by mafic rockswith MORB/BABB signatures. At Pyhäsalmi, the volcanicrocks are of transitional magmatic affinity, and the basaltsand basaltic andesites are tholeiitic and have Island ArcBasalts (IAB) signatures (Rasilainen et al. 2003; Mäki et al.2015). Similar conclusions were drawn by Roberts et al.(2003, 2004) from the Ruostesuo Zn-Cu and KangasjärviZn-Cu deposits, located at the southern extension of theVihanti-Pyhäsalmi belt. The altered rocks at the vicinity ofthe ore deposits in the Pyhäsalmi region (Kangasjärvi,Ruostesuo and Mullikkoräme) are mainly surrounded bysericite-cordierite and garnet-cordierite-anthophyllite assem-blages without any evidence of aluminous alteration(Roberts et al. 2003, 2004; Mäki et al. 2015). Thechemostratigraphy at the Pyhäsalmi deposit indicates the im-mediate protoliths to massive sulphide mineralisation as rhy-olites A and B of transitional magmatic affinity (Mäki et al.2015). At Pyhäsalmi, the original proximal part of the

hanging-wall sequence, tholeiitic mafic volcanic rock, wasremoved by fault displacement and is not in contact with themineralised stratigraphy (Mäki et al. 2015).

The Viholanniemi Zn-Cu-Pb-Ag-Au deposit host lithologyis very similar to that of Korpela including felsic and interme-diate volcanic rocks interlayered with volcaniclastic units ofvariable compositions. The Viholanniemi deposit is associatedwith a pyrite-quartz-sericite (± epidote) assemblage withquartz-carbonate veins. Nevertheless, this sericite-quartz zoneshows only minor Na2O depletion compared to similarsericite-quartz altered rocks at Korpela. This may suggest thatthe alteration intensity increases towards the Korpela succes-sion, from the sericite-quartz zone to the sericite zone, leadingto the more thorough breakdown of plagioclase and, therefore,stronger Na2O depletion. Makkonen (1991) suggested that themineralised carbonate-quartz veins at Viholanniemi could rep-resent a stringer zone of the deeper parts of a VMS hydrother-mal system. The quartz-sulphide or, less commonly, quartz-tourmaline veins have been documented in some Au-VMSdeposits in metamorphosed submarine volcanic settings(Dubé et al. 2007a). However, the Viholanniemi deposit doesnot exactly fulfil the requirements of gold-rich VMS depositclassification of Poulsen and Hannington (1995) having alower concentration of gold compared to combined basemetals (Zn + Cu + Pb wt%). The main differences betweenViholanniemi and Korpela are, for example, the differences inthe sulphide content and hydrothermal alteration. Furtherstudies are therefore required in order to discern their geneticrelation.

Clearly, a straightforward correlation cannot be made be-tween the Viholanniemi suite and the Vihanti-Pyhäsalmi belt.In this study, petrochemical evidence suggests that theKorpela succession has a different tectonic setting within theprimitive arc complex rocks in the RLSZ. Korpela rocks arepredominantly transitional to calc-alkaline volcanic orvolcaniclastic rocks with the presence of pervasive aluminousand sericitic alteration. The petrochemical assemblages atKorpela resemble that of the bimodal-felsic VMS type (e.g.Galley et al. 2007; Gibson et al. 2007) having the rhyolites ofFII-FIIIa chemical signatures (Fig. 10c) which are interpretedto have formed via partial melting of either continental oroceanic crust resulting from basaltic underplating duringrifting (Lesher et al. 1986; Barrett and MacLean 1999; Hartet al. 2004; Piercey 2011). In addition, the rhyolites havestrongly elevated HFSE contents (e.g. Zr at 350–550 ppm;Fig. 10f), similar to rocks commonly associated with a conti-nental rift or continental back-arc rift setting (Piercey 2007).Lawrie (1992) suggested an oceanic or back-arc setting for theVirtasalmi area with the hesitation of not having any evidencefor earlier felsic calc-alkaline arc complex rocks. In fact, thefelsic, calc-alkaline dominated, volcanic rocks that Lawrie(1992) did not identify may be located in the Viholanniemisuite. Zhang (2000) presents that the volcanics in the northern

Miner Deposita

part of the Viholanniemi area indicate an incipient arc riftsetting having detected MORB and BABB mafic rocks withhigh contents of compatible elements (TiO2) and subductionand within-plate volcanism-related multi-element distributionpatterns. Mafic volcanic rocks belonging to the Virtasalmisuite typically have pillow structures and pyroclastic textures(Kousa et al. 2018) suggesting a submarine volcanic environ-ment (Lawrie 1992, Zhang 2000). Other geological evidenceto support the rift setting (Vivallo and Claesson 1987) atKorpela are the presence of felsic to mafic dykes and tholeiiticbasalt-dominated lavas in the southern part of Viholanniemisuite. Felsic and mafic dykes are important because they pro-vide evidence of rift corridors: the dykes are supposed to haveexploitation of the same deep-seated pathways that circulatedhydrothermal circulation (e.g. Gibson et al. 1999; Galley1996, 2003; Campbell et al. 1981). Observations of the in situfragmentation of the volcaniclastic rocks provide evidence forrocks associated with subaqueous felsic domes andcryptodomes (McPhie et al. 1993). Despite the fact that nomassive sulphide deposit is yet to be found at Korpela, thetextural features from sulphide stringer zones provide evi-dence for replacement processes (e.g. Doyle and Allen 2003;Piercey 2015). These textural features are (1) the occurrenceof sulphides as interstices and open spaces or replacement ofgangue minerals, (2) volcaniclastic rocks and coherent lavasas intermediate host rocks and (3) a strong pipe-like alterationdomain and a weaker hanging-wall alteration zone. Theseobservations are preliminary and further studies are neededto confirm the sub-seafloor replacement-style massive sul-phide environment.

The proposed tectonic setting of volcanism is mainly basedon petrochemical assemblages with supportive geological ev-idence from drill core which means that a tectonic settingcannot be drawn to a final conclusion in this study.Generally, intra-continental back-arc environments are highlyprospective for VMS deposits, especially for gold-rich VMS(Galley et al. 2007; Dubé et al. 2007a; Caté 2016). The VMSdeposits in these types of settings occur in, mainly submarine,mafic bimodal to felsic bimodal and bimodal siliciclastic oro-genic and greenstone belts from Archean to Phanerozoic inage (Dubé et al. 2007a; Galley et al. 2007). Deposits are com-monly located adjacent to major crustal-scale faults and largesubvolcanic intrusions (Dubé et al. 2007a). Examples ofbimodal-felsic VMS districts are the PaleoproterozoicSkellefte and Bergslagen Districts in Sweden (e.g. Allenet al. 1996a; Allen et al. 1996b; Galley et al. 2007; Gibsonet al. 2007), the Palaeozoic Finlayson Lake District (e.g.Peters et al. 2007) and the Ordovician Bathurst Camp inCanada (van Staal et al. 2003). Gold-rich VMS deposits aretypically found in calc-alkaline centres and locallycharacterised by aluminous alteration interpreted as metamor-phosed advanced argillic alteration zones indicative of high-sulphidation conditions and formed in shallow-water

submarine equivalents to subareal epithermal deposits (e.g.Sillitoe et al. 1996; Hannington et al. 1999). Similar evolvedarc settings have also been suggested for SW Finland, at theHaveri Cu-Au (Tampere schist belt) and Iilijärvi Cu-Au-Zn(Orijärvi formation) (Eilu 2012). The bimodal Orijärvi forma-tion and overlying Kisko formation (ca. 1.90–1.88 Ga) withinUusimaa belt, SW Finland, are predominantly of calc-alkalineto tholeiitic affinity, and resemble semi-continuous rift-relatedsetting from primitive extension to evolved arc-type environ-ment (Latvalahti 1979; Mäkelä 1989; Väisänen and Mänttäri2002). Orijärvi formation hosts several auriferous VMS de-posits and occurrences (Mäkelä 1989). Dubé et al. (2007b)concludes that aluminous schists with anomalous Au and/orZn values in an intermediate to felsic, transitional to calc-alkaline volcanic or volcaniclastic rocks represent excellentexploration targets.

Subvolcanic intrusions and synvolcanic dykes

Subvolcanic intrusions in the rift or caldera systems havecommonly been interpreted as supplying heat to drive theconvective hydrothermal systems through synvolcanic deep-seated fault structures (Campbell et al. 1981; Galley 2003;Galley et al. 2007). The Saunakangas intrusion fits intoGalley’s (2003) prospective VMS-related subvolcanic intru-sion size range of 10–60 km2. The presence of foliation,miarolitic cavities, epidote alteration patches, transecting ofaplitic felsic dykes and xenoliths of mafic rocks in theSaunakangas intrusion (Fig. 3a–c) are all common featuresof VMS-associated subvolcanic intrusions (Galley 2003;Hannington et al. 2005). Most of the Saunakangas tonalitesamples, collected from different intrusive phases, showslightly different chondrite-normalised REE patterns com-pared to VMS-related subvolcanic intrusions elsewhere(ESM Fig. 4a, b) (Galley 2003; Ohtomaa 2014). The excep-tion to this is the tonalite sample 5, located at the porphyritictonalite phase close (< 200 m) to the Viholanniemi volcanicrocks (Fig. 2a and ESM Fig. 1). The extent of this particularintrusive phase is hard to estimate due to lack of outcrops (Fig.2). Sample 5 shows very similar REE signatures to the imme-diate felsic and intermediate rocks from Viholanniemi (ESMFig. 4c). The similar chondrite-normalised REE pattern indi-cates compositional matching to the associated volcanic suc-cession which is described elsewhere as a typical feature forshallow subvolcanic intrusion complexes (Piercey 2011).

Conclusions

The Korpela Cu-Zn-Au occurrence is situated in the centralpart of the Viholanniemi suite, in the Jäppilä-Virtasalmi block,which forms the southeastern end point of older (ca. 1920–1930 Ma) Svecofennian rocks. Korpela is within a structural

Miner Deposita

block of a bimodal, sub-alkaline, volcanic succession. It ischaracterized by HFSE-enriched (A-type) rhyolites of calc-alkaline affinity, rhyolites of FII-FIIa signatures and overlainand cross-cut by mafic rocks with MORB signatures. Thelithogeochemical signatures of the least-altered rocks are in-dicative of extensional and high-temperature felsic volcanicrocks which are critical for VMS Mineral Systems. The prox-imal presence of a subvolcanic tonalitic intrusion, lithologicalassemblages and trace element characteristics of felsic andmafic rocks suggest that Korpela, within the Viholanniemisuite, represents a bimodal, felsic dominated, mature arc riftof possible continental back-arc environment.

Based on detailed immobile-element methods, 12chemostratigraphic units and 21 chemical rock types can beidentified, ranging from basalt to rhyolite, at Korpela. Theleast-altered volcanic rocks define a common magmatic frac-tionation trend indicating a largely comagmatic volcanic groupwith dominantly acalc-alkaline affinity. Chemostratigraphy ofthe hosting succession, even where completely converted tosecondary metamorphic mineral assemblages, provides an im-portant tool for stratigraphic correlation with alteration types andthe sulphide-rich zone. The sulphides occur as stringers orientedparallel to the S2 foliation and are locally tightly folded andtransposed following the crenulation. The sulphide-bearing frac-tures subparallel to S2 are an expression of remobilisation andpresent mainly as Cu sulphide stringers within the Mg-Fe-Salteration type and Fe sulphide stringers within the K-Al-Mg-Fe-S alteration. The iron sulphides occur, predominantly, indacite A1 and rhyolite B1, whereas the Cu sulphides are con-centrated mainly with andesite A1. Volcanic rocks were affectedby different degrees of silicification, aluminium, sericitisationand chloritisation alteration at Korpela. During amphibolite fa-cies metamorphism at the regional D2 deformation stage, thehydrothermal mineralogy was transformed to mineral assem-blages characterised by varying amounts of quartz, muscovite,biotite, sericite, andalusite chlorite, garnet, chlorite, carbonate,staurolite, sillimanite, cordierite and accessory rutile, sillimaniteand tourmaline. Six chemical alteration types were recognized atKorpela: Mg-Fe-S, K-Al-Fe-(± S), K-Al-Mg-Fe-S, K, Si-K-Ca-(± S) and Ca-(± Na). The alteration types are spatially dis-tinguished zones that are interpreted to reflect VMS-style hydro-thermal alteration. The Mg-Fe-S alteration type is indicative ofMg-metasomatism. The K-Al-Mg-Fe-S and K-Al-Fe-(± S)types are characterised by strong aluminous alteration (Al-richphase) which is interpreted to portray advanced argillic alter-ation. The hanging-wall alteration is described by a sodiumand carbonate alteration assemblage.

Acknowledgements The author would like to express his most sincereappreciation to Pasi Eilu and Tero Niiranen for their constructive commentswhich improved the quality of the manuscript. Jukka Kousa is thanked forhis contributions and for sharing his wide knowledge of the Viholanniemiarea. Sami Niemi is thanked for geophysical maps and interpretations.Tuomo Stranius, Rauli Lempiäinen and Mauri Luukkonen are thanked

for their prospecting eye and excellent field assistance during the project.The author would also like to thank Esa Heilimo, Perttu Mikkola, Thair Al-Ani, Jouni Luukas, Erkki Luukkonen, Tapio Halkoaho, Hannu Makkonenand Asko Kontinen for their support and thoughtful discussions. My greatappreciation also goes to Denis Schlatter and an anonymous reviewer whoprovided thorough and thoughtful suggestions and comments which greatlyimproved the manuscript. The editorial comments by Bernd Lehmann arealso greatly appreciated.

Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, aslong as you give appropriate credit to the original author(s) and thesource, provide a link to the Creative Commons licence, and indicate ifchanges weremade. The images or other third party material in this articleare included in the article's Creative Commons licence, unless indicatedotherwise in a credit line to the material. If material is not included in thearticle's Creative Commons licence and your intended use is notpermitted by statutory regulation or exceeds the permitted use, you willneed to obtain permission directly from the copyright holder. To view acopy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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